"When the fierce leader of Carthage has been expelled, the hope and faith ofcourageous men will hold the republic together."

But why could it not also have a very slightly different meaning? :

"With the fierce leader of Carthage expelled, the hope and faith ofcourageous men will hold the republic together."

I.e. the first sentence is sort of a conditional statement (when this will have been done), whilst the second sentence is saying that the fierce leader has already been expelled so now the courageous men will definately be able to hold the republic together.

Slightly different, but certainly a different meaning, so why could the second translation not also be correct?

2)

Credunt magistram sapientem veritatem patefacturam esse.

The official translation for this sentence is:

"They believe that the wise teacher will reveal the truth."

So does this mean that patefacere is actually a deponent verb? If it was not a deponent verb, should the sentence not be written such:

"Credunt magistram sapientem veritatem patefecisse."

3)

His rebus pronuntiatis, igitur, eum esse hostem scivisti.

The official translation for this sentence is:

"When these things had been announced, therefore, you knew that he was an enemy."

But the first part of the sentence appears to be in the 2nd person plural conjuctive form (i.e. "you people announce"), not in the pluperfect passive form, which would be "prōnuntītī erant". So should the sentence not have been written as such:

1) You are correct; there is nothing preventing either of your readings from being correct. There is only one thing that can help you distinguish among the various options for translating an ablative absolute: the context. Granted, the sentences in Wheelock are generally not part of longer readings, so in this particular instance I suppose it's up to your preference. The point is that knowing the context (i.e., whether or not the dux has been expelled yet) will help you determine how to translate ablatives absolute in the future.

2) The form patefacturam esse is the future active infinitive (formed with the future active participle of the verb in question and the present infinitive of esse). The future passive infinitive, in this case, would be patefactamiri. The sentence "Credunt magistram sapientem veritatem patefecisse" would be translated "They believe that the wise teacher (has) revealed the truth."

3) The word pronuntiatis is, in this case, not the 2nd plural present indicative active, but rather the ablative plural of the perfect passive participle, pronuntiatus, -a, -um. The opening phrase, then, is an ablative absolute. The reason it appears to be translated as a pluperfect verb ("had been announced...") is that the main verb, scivisti, is perfect tense.

That was a very comprehensive answer. I now know that I have a lot more to learn when it comes to participles. The Wheelock's book condenses them all into a single chapter! (23)

And it barely mentions that they are all declinable! After having completed Ch. 23, I was under the impression that just the present participle was declinable.

Now, re-reading the chapter I see that it does briefly mention that the other three participles are declined like 'magnus', just before it goes into a detailed table of how to decline the present participle.

I recall dealing with similar confusions and frustrations when I first started Latin (using Wheelock) just about 9 years ago. Many times I convinced myself it just wasn't for me, but somehow I made myself persevere. I think everyone has those moments, so don't let it get to you! Keep at it, and all that stuff will eventually become second nature.